THE ETIOLOGY of chronic lymphocytic thyroiditis (Hashimoto's disease) is unknown. The theory that an autoimmune process is related to its pathogenesis was advanced by Roitt and co-workers1 when they demonstrated several types of antibody against the thyroid in the sera of patients with this disease. The finding of thyroid antibodies in the relatives of patients with chronic lymphocytic thyroiditis2 has suggested to some the possibility of genetic mechanisms acting in its causation, and the diagnosis of the disease in several members of the same family is considered compatible with this theory. Interest in this question of genetic predisposition increased with the demonstration of the disease in monozygotic female twins. Five sets of identical twins with chronic lymphocytic thyroiditis have been reported in the world literature.3-6 A sixth pair is now described.

Report of CasesA pair of 65-year-old female uniovular twins have been followed up for the